VARIATIONS IN RADAR COVERAGE 131 
ping, but because of lowered set performance there 
was no corresponding increase in fixed echo strength. 
The set then will appear to be in good operating con- 
dition, and the operator will be deceived about ranges 
of detection for craft flying above the duct. Equip- 
ment for checking set performance is not usually 
available in the field. The change in intensity of 
nearby fixed echoes may be, in some cases, a measure 
of set performance, but in the absence of rnore 
elaborate checks this method can be misleading and 
should not be relied upon entirely. 
Failure of detection of targets is not necessarily 
due to weather influences. Electrical failure of the 
set or inadequate adjustment may be the difficulty 
and may be far more troublesome to identify than 
meteorological effects which should not be used as 
a “scapegoat” to.be indiscriminately blamed for 
poor coverage. 
METEOROLOGICAL FACTORS 
The atmosphere is responsible for bending and 
duct formation. To understand the ‘why’’ of non- 
standard ranges of radar and radio with respect to 
the weather, it is necessary to consider the meteoro- 
logical factors involved. 
The strong refraction which results in guided 
propagation is caused by a rapid decrease of index 
of refraction with height within: certain layers. The 
decrease depends upon distribution of moisture and 
temperature in the atmosphere, particularly in the 
lowest few hundred or thousand feet. Normally the 
temperature decreases with height in the atmosphere 
(at a rate of about 2°C per 1,000 ft), and the mois- 
ture decreases gradually with height. Under these 
conditions the propagation is of the standard type. 
Temperature may sometimes increase with height 
for a few hundred or thousand feet above ground 
and then, at greater heights, begin to decrease again. 
The vertical increase of temperature is called a tem- 
perature inversion. Sometimes a layer of moist air 
is found near the ground, and the air overlying it is 
very dry. There is then a rapid decrease of moisture 
over a short vertical distance: in other words there 
1500) 
1000 
500 
ALTITUDE IN FEET 
MIXING RATIO INCREASE 
Ficurr 7. Moisture variation aloft. 1. Moisture distri- 
bution with height in standard moist atmosphere. 2. 
Example of sharp moisture lapse (dry air overlying 
moist air) conducive to guided propagation. Mixing 
ratio is amount of moisture in a unit weight of dry air 
expressed as grams of water per kilogram of dry air. 
is a pronounced moisture lapse (see Figure 7). A 
moderate or strong moisture lapse almost always will 
produce trapping, but a temperature inversion 
(except at low temperatures) will lead to trapping 
only if the moisture distribution is favorable. A 
combination of both effects within the same layer 
usually will produce trapping. 
The meteorological conditions to be found over sea 
and over land are quite different and must be con- 
sidered separately. 
OVER SEA 
When warm, dry air flows over colder water, a 
temperature inversion will be established, and there 
will be evaporation into the lowest layers of the air, 
thus creating conditions of pronounced trapping. 
This weather condition is one of the most common 
causes of guided propagation. An example in point 
is the Mediterranean, which to the south, east, and 
west is surrounded by dry land masses producing a 
flow of dry, warm air over the water when the winds 
AIR OVER AFRICA 
TEMPERATURE 
DISTRIBUTION 
AIR FROM SAHARA 
VERY DRY 
A 
\ 
\ 
A 
4 
\ 
LY 
L) 
A 
i) 
A 
A 
HEIGHT IN FEET 
LAND TEMP 
AFTER PASSAGE ACROSS THE MEDITERRANEAN 
MOISTURE 
DISTRIBUTION 
\ TEMPERATURE 
\ DISTRIBUTION 
i 
\ 
v 
\ 
\ 
) 
y) 
HEIGHT IN FEET 
7 sea TEMP 
TEMPERATURE 
— UP MIXING RATIO —=INCREASE 
Ficure 8. Modification of air from Sahara Desert in 
passing over the Mediterranean. 
blow from these directions (see Figure 8). Similar 
conditions are often caused by westerly winds. blow- 
ing from land to sea across the eastern boundary of 
a continent. Land and sea breezes may influence 
radar operation along a coast line. The wind direc- 
tion at a coast is often an important factor in deter- 
mining propagation conditions and should be closely 
watched. Whenever unusual propagation is observed 
by coastal radar stations, a record of prevailing winds 
at the time is very helpful in determination of future 
expected performance. 
Over Lanp 
Temperature inversions are produced mainly by 
nocturnal or night cooling of the ground (see Figure 
9). Trapping may occur when the moisture distribu- 
